Horizontal injection molding machines mainly comprise a mold clamping apparatus for clamping a mold, and an injecting apparatus for injecting a resin material into the clamped mold. The mold is opened when a molded product is taken out. Generally, mold opening is performed by operating a mold opening/closing actuator which is provided separately from a mold clamping cylinder.
The mold opening/closing actuator has a longer stroke and a smaller cylinder diameter compared to the mold clamping cylinder. When the molded product is large in size, a large mold opening force is required. To increase the mold opening force, it is considered that the cylinder diameter of the mold opening/closing actuator is increased, which, however, makes the mold opening/closing actuator expensive. As one of other measures, a technique is disclosed which allows an initial stage of a mold opening process to be performed by the action of the mold clamping cylinder without increasing the cylinder diameter of the mold opening/closing actuator, for example, in Japanese Patent (JP-B) No. 5084797.
The prior art invention disclosed in JP 5084797 B will be described below with reference to FIG. 9. As shown in FIG. 9, a conventional mold clamping cylinder 100 includes a large-diameter piston 101, a medium-diameter piston 102 disposed adjacent to the large-diameter piston 101, a piston rod 103 extending from the medium-diameter piston 102, a small-diameter rod 104 extending from the large-diameter piston 101, a case 105 with a bottom for movably receiving therewithin the large-diameter piston 101 and the small-diameter rod 104, and a lid 106 for covering an opening of the case 105.
In a mold clamping process, pressure oil is supplied to a large-diameter oil chamber 111 formed behind the large-diameter piston 101 and a small-diameter oil chamber 112 formed behind the small-diameter rod 104, whereby the piston rod 103 is moved forward. In an initial stage of a mold opening process, pressure oil is supplied to a medium-diameter oil chamber 113 formed in front of the medium-diameter piston 102, whereby the piston rod 103 is moved rearward. The medium-diameter piston 102 generates a sufficiently large mold opening force. Since the medium-diameter piston 102 has a smaller diameter compared to the large-diameter piston 101, the medium-diameter oil chamber 113 has a relatively small diameter, and thus the piston rod 103 is moved rearward at a speed faster than that in the mold clamping process.
However, since the medium-diameter piston 102 and the medium-diameter oil chamber 113 need to be provided, the structure of the conventional mold clamping cylinder 100 is inevitably complicated. Additionally, four oil passages 114, 115, 116, 117 are needed. Thus, in comparison to a common cylinder having a single piston and two oil passages, the conventional mold clamping cylinder 100 becomes complicated and expensive.
To respond to a demand for cost reduction of the mold clamping apparatus, an improved mold clamping apparatus is required which is capable of boosting the mold opening force in the initial stage of the mold opening process while suppressing cost increase.